Medical equipment for radiation therapy treats tumorous tissue with high-energy radiation. The amount of radiation and its placement must be accurately controlled to ensure both that the tumor receives sufficient radiation to be destroyed, and that the damage to the surrounding and adjacent non-tumorous tissue is minimized.
In external source radiation therapy, a radiation source external to the patient treats internal tumors. The external source is normally collimated to direct a beam only to the tumorous site. The source of high-energy radiation may be from linear accelerators as x-rays, or electrons, protons, neutrons or any other form, in the range of 2-300 MeV, or gamma rays from highly focused radioisotopes such as a Co60 source having an energy of 1.25 MeV.
Typically, the tumor will be treated from several different angles with the intensity and shape of the beam adjusted appropriately. The purpose of using multiple beams, which converge on the site of the tumor, is to reduce the dose to areas of surrounding non-tumorous tissue. The angles at which the tumor is irradiated are selected to avoid angles which would result in irradiation of particularly sensitive structures near the tumor site. The angles and intensities of the beams for a particular tumor form a treatment plan for that tumor.
More-advanced, highly accurate modalities of radiation delivery have been developed to further customize a treatment plan to conform dose to a target region while limiting dose outside that target. Such modalities modulate individual “beamlets” of radiation within each beam so that all beamlet from all beams, in sum, create an optimal plan. Beamlet modulation may be achieved in many ways, including: temporal motion of multi-leaf collimators during delivery, rotational beams with moving collimators, solid physical modulator that optimizes the beam through a precision milled device, and non-coplanar robotic arms delivering many small, distinct beams from many angles.
In order to take advantage of the improved accuracy in dose placement offered by such optimized radiation planning and delivery systems, the radiation treatment plan may be based on a digitized virtual model of the patient's anatomy, which is built using volumetric medical imaging. The most common in volumetric medical imaging modalities are computed tomography (“CT”) and magnetic resonance imaging (“MRI”) As is known in the art, a CT image is produced by a mathematical reconstruction of many projection images obtained at different angles about the patient to provide an image of “slices” or planes throughout the patient.
Using the stack of CT images, the radiologist views the tumorous area and determines the beam angles and intensities (identified with respect to the tumor image) which will be used to treat the tumor. Different regions may be defined within each slice plane of a series of CT images in a process known as “segmentation.” For example, regions to receive high-dose may be defined on each CT image by creating segmentation of “target areas” in that image, whereas regions that should be spared radiation because of radiation sensitivity may also be segmented in that 2D image to help guide the treatment planner on where to avoid high doses. Additional areas of segmentation may also be defined with different dose levels. This process is repeated for multiples adjacent CT images to provide a three-dimensional segmentation.
The segmentation may be done manually by clinicians (i.e. a trained dosimetrist may segment the critical sparing organs, while a physician may define the target regions) or by using various automatic segmentation programs such as those commercially available from Varian Medical Systems, Inc. of California, USA under the Eclipse “Smart Segmentation” trade name, from Royal Philips Electronics of the Netherlands in their Pinnacle system under the trade designation “Model-based Segmentation,” and from CMS, Inc of Missouri, USA under the trade name “Atlas-based Autosegmentation.” The results of the segmentation are stored in segmentation files, currently under a DICOM standard as DICOM-RT Structure Set files. These files contain point data defining the periphery of a volume in multiple parallel planes or slices.